1. Field of the Invention
The present invention relates to an apparatus and a method for measuring bio-information, and more particularly to an apparatus and a method for measuring information about the skin and subcutaneous fat by using light.
2. Description of the Related Art
Various methods for measuring body fat or the thickness of subcutaneous fat have been known to the general public. Methods for measuring the thickness of subcutaneous fat include a method for direct measurement using calipers, a method using ultrasonic waves, a method using Computerized Tomography (CT), Magnetic Resonance Imaging (MRI), and the like. The method for directly measuring the thickness of subcutaneous fat using calipers is inaccurate and inconvenient, and causes pain. The method for measuring the thickness of subcutaneous fat by using ultrasonic waves, or CT, or MRI require expensive equipment and experts. Also, since it is inconvenient to measure the thickness of subcutaneous fat because of the need for devices for implementing the previously described methods, research on measurement methods using light have been performed.
One method for measuring body fat or the thickness of subcutaneous fat by using light corresponds to illuminating light onto the surface of the skin of a living body, and then detecting light radiating from the surface of the skin due to multiple scattering and converting the detected light to an electrical signal. This method has advantages in that it is non-invasive, and has a short measurement time. Also, since an apparatus for implementing this method has a small size, the apparatus has an advantage that it can measure the thickness of subcutaneous fat on occasion.
FIG. 1 is a view illustrating an apparatus for measuring the thickness of subcutaneous fat according to the prior art. Biological tissue 130 has a structure in which respective layers corresponding to muscle 132, subcutaneous fat 134, and skin 136 having a thickness of 0.5 to 4 mm, are laminated in that order, and the skin 136 can be subdivided into a horny layer, the outer skin, and the true skin. The above apparatus 100 includes a Light Emitting Diode (LED) 110 and a PhotoDiode or PhotoDetector (PD) 120. The LED 110 and the PD 120 are separate from each other, and are arranged on the surface of the skin 136. If the LED 110 irradiates light having a wavelength in the band of near infrared on the surface of the skin 136, some light travels in a direction from the surface of the skin 136 to the muscle 132, some of the light is directed to the surface of the skin 136 due to multiple scattering. Most of the light is absorbed by the muscle 132. The PD 120 detects light radiating from the surface of the skin 136 and converts it to an electrical signal.
FIG. 2 is a graph illustrating a change in the output of the PD 120 as a function of the thickness of the subcutaneous fat 134 according to separation distances from the LED 110. The X axis represents the thickness of the subcutaneous fat 134 in millimeters (mm), and the Y axis represents the output voltage of the PD 120 in Volts (V). Herein, an ‘SD’ represents a Separation Distance between the LED 110 and the PD 120. FIG. 2 depicts output curves corresponding to cases where SDs equal 5 mm (expressed in a solid line including small squares), 10 mm (expressed in a solid line including small circles), and 20 mm (expressed in a solid line including small triangles),respectively. To examine the output curve relevant to each SD, it can be recognized that the thicker the subcutaneous fat 134 the more an output of the PD 120 increases toward becoming saturated. Also, it can be determined that the more distant the SD becomes, the wider the range of the thickness of the subcutaneous fat 134 that can be measured.
U.S. Pat. No. 4,850,365, entitled “Near Infrared Apparatus and Method for Determining Percent Fat in a Body”, invented by Rosenthal et al., discloses technology in which light having a single wavelength is irradiated on the surface of the skin, and then, percent body fat is measured by detecting light radiating from the surface of the skin due to multiple scattering within the subcutaneous fat layer.
U.S. Pat. No. 4,633,087, entitled “Near Infrared Apparatus for Measurement of Organic Constituents of Material”, invented by Rosenthal et al., discloses technology in which multiple rays of light having wavelengths different from one another are irradiated on the surface of the skin, and then, percent body fat is measured by detecting rays of light radiating from the surface of the skin due to multiple scattering within the subcutaneous fat layer.
However, since not only the thickness of subcutaneous fat but also the color and the thickness of the skin affects the output of the PD, the prior technologies of bio-information measurement have problems in that an error in percent body fat caused by the color and the thickness of the skin, or an error in the thickness of subcutaneous fat cannot be effectively corrected.